1. Field of the Invention
The present invention relates to the production of ethanol. In particular, the present invention concerns a new process for conversion of fibrous lignocellulosic material to ethanol by fermentation.
2. Description of Related Art
Conversion of biomass to fuel energy has received growing attention as a means of replacing energy derived from fossil fuels. Of the liquid biofuels (ethanol, methanol, fatty acid methyl ester), ethanol has long proven history and environmental advantages. It can be produced from a variety of raw materials. Traditionally, ethanol has been produced from starch or sugar based agricultural products, but today the focus is on different agricultural and forestry residues or side streams from forest industries. A very significant environmental advantage of ethanol production is that there is low generation of CO2, provided that the raw material is driven from renewable waste residues or wood. At present, the cost of ethanol produced from lignocellulose containing raw materials is still too high for several reasons. Among the barriers are the high costs of the lignocellulose conversion technologies, the low concentration and yield of ethanol, as well as the low production rates, which all increase the costs of ethanol produced from lignocellulosics, as compared to ethanol produced from starch or sugar based raw materials.
Conversion of wood or agriculture derived lignocellulosic materials into sugars and further to ethanol is a complex process involving several steps (pretreatment, potential separation of solids, hydrolysis of cellulose, ethanol production from cellulose and hemicellulose and distillation of ethanol). Depending on the raw material, different types of pretreatment techniques are needed. A pretreatment step is usually needed to improve the hydrolyzability of the cellulosic part. The objective of the pretreatment is to render the biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient production of sugars from cellulose and hemicellulose. The goals of the pretreatment are to remove and separate hemicellulose from cellulose, to disrupt and remove the lignin sheath, to decrease the crystallinity of cellulose, to increase the accessible surface area of cellulose, and to increase the pore size of cellulose to facilitate the penetration of hydrolysing agents (Chang and Holtzapple, 2000).
Detailed descriptions of various pretreatment technologies are available (reviewed e.g. by Hsu et al, 1996). Of the various pretreatment options, steam explosion (with sulphuric acid impregnation and with sulfur dioxide) is one of the most extensively studied methods (Chandrakant and Bisaria, 1998).
The maximum digestibility of cellulose usually coincides with complete hemicellulose removal. Therefore, in efficient pretreatment methods, most of the hemicellulose is solubilized and forms a soluble fraction containing mainly hemicellulose derived sugars (referred to as a “hemicellulose filtrate”). The crude hemicellulose filtrate from the pretreatment contains usually various degradation products of lignocellulose. These may be lignin and sugar decomposition products, including furfural, hydroxymethyl furfural and formic and acetic acid. Most of these components are toxic to enzymes and microorganisms slowing the subsequent hydrolysis and fermentation process. A number of different detoxification methods have been investigated (Gong et al. 1999). Neutralization with lime, charcoal treatment and different adsorption resins are among the methods studied. Inhibitors in the hemicellulose filtrate have been shown to severely decrease both the hydrolysis and fermentation rates.
The hydrolysis processes of the cellulosic part may be based either on acid or enzymes. The major disadvantages of the enzymatic hydrolysis are that the process is quite slow and the enzyme costs are still too high (Kaylen et al, 2000). Generally, the hydrolysis yields depend on the type and pretreatment of the substrate, type and dosage of the enzyme and the hydrolysis time. Most experiments have been carried out at low raw material consistencies due to the amount of inhibitory compounds in the substrate derived from the pretreatment stage.
There are essentially two different types of processes that can be used to convert cellulose (and hemicellulose) to ethanol. These are the separate hydrolysis and fermentation (SHF) and the simultaneous saccharification and fermentation (SSF). The latter process has been also extended to contain simultaneous saccharification and hemicellulose fermentation (SSHF), and is also referred as simultaneous saccharification and cofermentation (SSCF). Among various cellulose bioconversion schemes, the SSF seems to be the most promising approach to biochemically convert cellulose to ethanol. Industrial ethanol production is traditionally carried out by yeast, which is a well known robust organism. New strains (either yeasts or bacteria) have been engineered to efficiently utilize all the sugars derived from the lignocellulosic raw material. Utilization of all sugars, including the hemicellulose derived pentoses and all hexoses, is essential for economical production of ethanol.
The hydrolysis conditions used in a separate hydrolysis process (SHF) are determined by the optimum conditions of the enzymes (mostly fungal cellulases having a maximum activity at 50° C. and at a pH in the range from 4 to 5). The main advantage of a separate hydrolysis stage is that the hydrolysis is carried out at the optimum temperature of the enzymes, and the separate fermentation at the optimum of the yeast, about 30° C. The major disadvantage is that the sugars released in the hydrolysis severely inhibit the cellulase activity during hydrolysis. This can be at least partially overcome by increasing the beta-glucosidase activity in the preparation used (by adding separate enzyme or by using an overproducing strain). The cellulase loadings usually range from 10 to 20 FPU/g of substrate (or cellulose), and beta-glucosidase is supplemented. Usually the sugar concentrations produced are quite low due to the low amount of dry matter in the hydrolysis. Yields (from the sugars) are usually higher in more dilute systems, where end product inhibition is minimized. Long reaction times also make higher ethanol yield and concentration possible.
In the simultaneous saccharification and fermentation process (SSF), the saccharification of cellulose to glucose with cellulases and the subsequent fermentation of glucose (and pentoses) to ethanol takes place in the same reactors. According to present process schemes, all reactants (cellulose, enzymes and fermenting organism) have been added at the same time. One of the most important requirements of the SSF process is the compatibility of the saccharification and fermentation systems with respect to temperature (below 37° C.), pH and substrate concentration. The main advantages offered by SSF include enhanced rate of cellulose hydrolysis due to uptake (by yeast) of sugars inhibiting cellulase activity and decreased requirement of aseptic conditions. The disadvantages are the differences in optimal conditions for hydrolysis and fermentation. Using the whole material; both the solid cellulose and hemicellulose filtrate simultaneously for fermentation instead of only the filtrate has shown advantages, for example lactic acid formation is reduced (Stenberg et al. 2000).